An analysis of mitochondrial messenger RNA metabolism in Saccharomyces cerevisiae during repression or derepression of respiratory activity is proposed. Purification of mitochondria free of cytoplasmic RNA species and a general characterization of mitochondrial poly(A)-containing RNA constitutes the first phase of the investigation. A complementary DNA probe (cDNA) will then be prepared and used for titration of changes in the amount of mitochondrial mRNA in cells undergoing changes in respiratory activity. Finally, an assessment will be made of the feasibility of isolating the specific mRNA for subunit II of cytochrome c oxidase.